1. Field of the Invention
The present invention relates to a sheet material holding device for holding sheet material in a wound state on a drum by clamping a winding-direction end of the sheet material wound around a peripheral surface of the drum to the peripheral surface of the drum.
2. Description of the Related Art
As apparatus for exposing printing plates, technology has been developed in which, using sheet-like recording materials, and particularly a printing plate comprising a support having disposed thereon a photosensitive layer, an image is recorded with a laser beam or the like directly on an emulsion surface that is a recording layer of the printing plate. With such technology, it has become possible to quickly record an image on a printing plate.
In automatic printing plate exposure apparatus employing technology for recording an image on a printing plate, the printing plate is sent to an exposure section with a flat plate being used as a support surface. The printing plate is received in the exposure section, positioned into a predetermined position, and exposed.
In the exposure section, a rotary drum is used to support the printing plate. The sent printing plate is wound around a peripheral surface of the rotary drum and held thereat. The rotary drum rotates in the main scanning direction. An image is exposed on the printing plate by moving a recording head along the axis of the rotary drum (i.e., sub-scanning).
A clamp is generally used to hold the leading and trailing ends of the printing plate to the rotary drum.
The clamp includes a clamp body that pivots in the peripheral direction of the rotary drum, with a column attached to the rotary drum serving as a fulcrum. The clamp body is urged by an urging device such as a spring to a clamping position at which a clamping portion disposed at one end of the clamp body is brought into contact with the peripheral surface of the rotary drum.
When the printing plate is sent to the rotary drum, an upper surface of the other end of the clamp body is pressed by extending a telescopic rod, whereby the clamping portion is separated, counter to the urging force of the urging device, from the peripheral surface of the rotary drum to a released position. In this state, the printing plate can be inserted between the clamping portion and the peripheral surface of the rotary drum. The telescopic rod is thereafter retracted, whereby the clamping portion is returned to the clamping position by the urging force of the urging device and holds the printing plate.
However, when the clamp body rotates with the column as a fulcrum, the vector of the force of extension/contraction, and mainly the force of extension, of the rod includes a directional force that causes the rotary drum to rotate. Due to this rotational force, the rotary drum is not fixed in a predetermined position and occasionally oscillates. Because the rotary drum must therefore be fixed by an additional mechanism, the structure of the apparatus becomes complicated, which results in stretching out working time until the printing plate is held.
In view of the aforementioned circumstances, it is an object of the present invention to provide a sheet material holding device capable of securely holding a sheet material, without fixing a rotary drum, and shortening holding operation time.
A first aspect of the invention is a device for holding sheet material in a wound state around a peripheral surface of a rotary drum, the device comprising: a column disposable at a predetermined position on the peripheral surface of the rotary drum; a clamp body pivotable about the column, the clamp body including one end comprising a clamping portion for clamping the sheet material to the peripheral surface of the rotary drum, the clamping portion being movable into at least clamping and released positions; an urging device for urging the clamping portion of the clamp body into the clamping position; and a clamping unit disposable spaced away from the rotary drum and including a linearly movable rod, which the clamping unit moves to contact an end of the rod with the clamp body and counters urging force of the urging device to thereby pivot the clamp body into the released position, wherein a locus of the linear movement of the rod and a position of contact between the clamp body and the rod are such that a straight line C and an axial line D of the rod intersect at a substantially right angle, the intersection being between a straight line A and a straight line B, the straight line A connecting a clamp body pivot point Y in the column and a contact point X between the end of the rod and the clamp body when the clamping portion is in the clamping position, the straight line B connecting the pivot point Y and a contact point Z between the end of the rod and the clamp body when the clamping portion is in the released position, the straight line C passing through the pivot point Y and being parallel to a line tangential to the peripheral surface of the rotary drum at a point where a line of shortest distance between the pivot point Y and the peripheral surface of the rotary drum intersects with the peripheral surface of the rotary drum.
According to the first aspect of the invention, the rod of the clamping unit is extendable so as to abut and press against the clamp body. Due to this arrangement, when the clamp body is pivoted about the column, the extending direction of the rod normally faces towards an axial center of the drum.
While the rod forward end is in contact with the clamp body, if the movement direction of the rod did not meet the axial center of the drum, a force for rotating the drum would be generated.
Therefore, according to the first aspect of the invention, a straight line C and an axial movement line of the rod intersects substantially at right angles with each other between a straight line A and a straight line B. Here, the straight line A connects a contact point between the rod forward end and the clamp body in a state that the clamping portion is in the clamping position with a rotational center of the column in the clamp body. The straight line B connects a contact point between the rod forward end and the clamp body in the state that the clamping portion is in the releasing position respectively with the rotational center of the column in the clamp body. The straight line C passes through the rotational center of the column of the clamp body and is parallel with drum tangent line at an intersection between a radius of the rotational center and the drum peripheral surface. As a result, a scope (widening angle) of directions of unnecessary force can be leveled and minimized with respect to entire range of pivot of the clamp body. Thus, defects due to rotating the drum by the press force of the rod can be eliminated.
Such arrangement can be carried out by adjusting a linear moving locus of the rod of the clamping unit and the contact position between the clamp body and the rod.
A second aspect of the invention is a device for holding sheet material in a wound state around a peripheral surface of a rotary drum, the device comprising: a column disposable at a predetermined position on the peripheral surface of the rotary drum; a clamp body pivotable about the column, the clamp body including one end comprising a clamping portion for clamping the sheet material to the peripheral surface of the rotary drum, the clamping portion being movable into at least clamping and released positions; an urging device for urging the clamping portion of the clamp body into the clamping position; and a clamping unit disposable spaced away from the rotary drum and including a linearly movable rod, which the clamping unit moves to contact an end of the rod with the clamp body and counters urging force of the urging device to thereby pivot the clamp body into the released position, wherein a locus of the linear movement of the rod and a position of contact between the clamp body and the rod are such that a straight line A intersects at a substantially right angle a locus E of movement of a contact point X at the time of movement of the clamping portion between the clamping and the releasing positions, the straight line A connecting a clamp body pivot point Y in the column and the contact point X between the end of the rod and the clamp body when the clamping portion is in the clamping position.
According to the second aspect of the invention, similarly to the first aspect, the rod of the clamping unit is extendable so as to abut and press against the clamp body. Due to this arrangement, when the clamp body is pivoted about the column, the extending direction of the rod normally faces towards an axial center of the drum.
The rod forward end abuts against the clamp body when the clamping portion is in the clamping position. From this state, the clamp body starts pivoting. When the stopping clamp body due to the inertial law is to be moved, the magnitude of force reaches its peak. Once the clamp body starts pivoting, this peak magnitude of force is excessive for the clamp body.
Therefore, according to the second aspect, a straight line intersects substantially at right angles with a movement locus of a contact point. The straight line connects the contact point touching the rod in the clamping position of the clamp body. As a result, the unnecessary force can be directed to the axial center of the drum, and thus defects due to rotating the drum by the press force of the rod can be eliminated.
Such arrangement can be carried out by adjusting the linear moving locus of the rod of the clamping unit and the contact position between the clamp body and the rod.